1) Field of the Invention
The present invention relates to an optical switch device which switches optical signals for WDM (Wavelength Division Multiplex) by selecting wavelengths of the optical signals.
2) Description of the Related Art
The optical communication networks can constitute a core to form a base of a communication network, and it is desired that the services of the optical communication networks become available in wider areas and further sophisticated. In particular, development of WDM techniques constituting a core technology for constructing optical communication systems is rapidly proceeding. The WDM is a technique in which a plurality of signals are concurrently transmitted through a single optical fiber by multiplexing light having different wavelengths (colors).
In the case where a photonic network is constructed by using WDM, optical crossconnection (OXC) as a technique for switching optical signals is used for efficiently utilizing wavelength resources.
FIG. 16 is a diagram schematically illustrating an example of the optical crossconnection. In the system illustrated in FIG. 16, an OADM (Optical Add-drop Multiplexer) unit 101 and an optical switch unit 102a are connected through an optical-fiber route R1, and the optical switch unit 102a and an OADM unit 103 are connected through an optical-fiber route R2. The OADM unit 101 comprises an optical switch unit 101a, an optical combiner 101b, and an optical splitter 101c, and the OADM unit 103 comprises an optical switch unit 103a. Although not shown, the optical switch unit 103a also comprises an optical combiner and an optical splitter.
The optical combiner 101b in the OADM unit 101 multiplexes optical signals having different wavelengths, and outputs the multiplexed signals to the optical switch unit 101a. (That is, the optical combiner 101b and the optical switch unit 101a add the above optical signals to signals flowing through the optical-fiber route R1.) The optical switch unit 101a performs switching of the signals outputted from the optical combiner 101b and signals flowing through the optical-fiber route R1 according to their wavelengths so that the switched signals are outputted to the optical-fiber route R1 and the optical splitter 101c. The optical splitter 101c optically demultiplexes the multiplexed signals outputted from the optical switch unit 101a to the optical splitter 101c, and separately outputs the demultiplexed signals for respective wavelengths which the demultiplexed signals have. (That is, the optical switch unit 101a and the optical splitter 101c drop the above optical signals from the optical-fiber route R1.) The optical switch unit 102a performs switching of optical signals received from the optical-fiber routes R1 and R2. The OADM unit 103 performs operations of switching, adding, and dropping optical signals in a similar manner to the OADM unit 101.
In the example illustrated in FIG. 16, optical signals having the wavelengths λA, λB, λC, and λD are outputted from the optical switch unit 101a onto the optical-fiber route R1, optical signals having the wavelengths λa, λb, λc, and λd are outputted from the optical switch unit 103a onto the optical-fiber route R2. At this time, the wavelength band assigned to optical signals having the wavelength λA, the wavelength band assigned to optical signals having the wavelength λB, the wavelength band assigned to optical signals having the wavelength λC, and the wavelength band assigned to optical signals having the wavelength λD are respectively identical to the wavelength band assigned to optical signals having the wavelength λa, the wavelength band assigned to optical signals having the wavelength λb, the wavelength band assigned to optical signals having the wavelength λc, and the wavelength band assigned to optical signals having the wavelength λd. For example, both the wavelength band assigned to optical signals having the wavelength λA and the wavelength band assigned to optical signals having the wavelength λa are a wavelength band to which the wavelength of 1,550 nm belongs. However, information conveyed by optical signals having the wavelength λA is generally different from information conveyed by optical signals having the wavelength λa.
The optical switch unit 102a performs a switching operation in which optical signals on the optical-fiber route R1 and optical signals on the optical-fiber route R2 in each of at least one of the wavelength bands are exchanged. That is, information in each wavelength band is exchanged. In the situation illustrated in FIG. 16, the optical switch unit 102a exchanges optical signals at the wavelengths λA and λa, and optical signals at the wavelengths λD and λd. After the switching operation, the optical switch unit 102a outputs optical signals at the wavelengths λa, λB, λC, and λd onto the optical- fiber route R1, and optical signals at the wavelengths λA, λb λc, and λD onto the optical-fiber route R2.
FIG. 17 is a diagram illustrating a conventional construction of the optical switch unit 102a. The optical switch unit 102a of FIG. 17 comprises optical splitters 102a-1 and 102a-2, optical combiners 102a-3 and 102a-4, and 2×2 switches SW1 to SWn. Each of the optical switch units 101a and 103a also comprises elements basically similar to the optical switch unit 102a. 
The input port of the optical splitter 102a-1 is connected to an end R1in of the optical-fiber route R1 for receiving optical signals from the optical-fiber route R1, and the input port of the optical splitter 102a-2 is connected to an end R2 in of the optical-fiber route R2 for receiving optical signals from the optical-fiber route R2. The output port of the optical combiner 102a-3 is connected to an end R1out of the optical-fiber route R1 for outputting optical signals onto the optical-fiber route R1, and the output port of the optical combiner 102a-4 is connected to an end R2out of the optical-fiber route R2 for outputting optical signals onto the optical-fiber route R2.
The optical combiners are realized by array waveguide gratings (hereinafter referred to as AWGs). The AWGs are normally formed of optical circuitry using optical waveguides made of quartz-based glass, and are widely used in the WDM systems since the AWGs are suitable for mass production. The AWGs can separate optical signals having a plurality of wavelengths, and output the optical signals into a plurality of waveguides provided in correspondence with the plurality of wavelengths, or can combine optical signals having a plurality of wavelengths in a single waveguide and output the combined optical signals from the single waveguide.
As understood from FIG. 17, devices such as the optical switch unit 102a which have the function of selecting wavelength components directed to at least two different fiber routes (e.g., the optical-fiber routes R1 and R2) need four AWGs and n 2×2 switches, where n is the number of wavelengths. That is, in the case where switching of a signal in which n wavelengths are multiplexed is performed, n 2×2 switches are needed.
According to a conventionally proposed technique for an optical switch which has the function of selecting wavelength components (for example, as disclosed in Japanese Unexamined Patent Publication No. 2002-72157, paragraph Nos. 0064 and 0065 and FIG. 15), a wavelength-variable filter is formed by varying the refraction indexes of waveguides in an AWG at a predetermined rate, and applied to an optical switch.
In recent years, the explosive spread of use of the Internet and some other factors have led to steady increase in the transmission rates. In this situation, the dense WDM (DWDM) is receiving attention and development of the DWDM is proceeding. The DWDM can realize tens to hundreds of wavelength channels through a single optical fiber in such a manner that each of the wavelength channels has a transmission rate of 600 Mbps to 10 Gbps, and a bundle of a very great amount of data is transmitted at a total transmission rate on the order of 1 Tbps.
However, when optical crossconnection is performed in a photonic metro network using DWDM, optical signals in which a very great number of wavelengths are multiplexed are handled, and therefore the number of channels for which switching processing is to be performed greatly increases.
In addition, in order to realize optical crossconnection in a DWDM system handling hundreds of wavelength channels by using the optical switch unit 102a having the construction illustrated in FIG. 17, it is necessary to use large-scale AWGs which can combine and split hundreds of wavelengths and have great dimensions, and hundreds of 2×2 switches.
Therefore, in the systems using the conventional optical switch unit 102a as illustrated in FIG. 17, the size and cost of equipment increase, so that it is impossible to construct a photonic network which is economical and highly operable.
Further, it is possible to mechanically realize optical switching with switch cells using microelectromechanical elements such as the MEMS (MicroElectroMechanical Systems). However, complicated control and great equipment size required by the use of the high-density optical switching elements make downsizing and cost reduction difficult.